1. Field of the Invention
This invention relates to a method to epitaxially grow a III-V nitride film, particularly AlxGayInzN (x+y+z=1) film on a given substrate using a Metal Organic Chemical Vapor Deposition (MOCVD) method and an apparatus for the same method.
2. Related Art Statement
In opto-electronic devices such as light-emitting diodes, laser diodes or photodiodes, it is proposed that III-V nitride films having their compositions of AlxGayInzN(X+Y+Z=1) are epitaxially grown on a given substrate made of sapphire single crystal, for example. Up to now, the epitaxial growth of the AlxGayInzN film has been performed using a MOCVD method or recently, a Hydride Vapor Phase Epitaxy (HVPE) method.
In the case of making a GaN film using a HVPE method, first of all, a substrate made of sapphire single crystal is set into a reactor in which a gallium metallic material is charged. Then, a hydrochloric acid gas is introduced into the reactor and reacted with the gallium metallic material, to generate a hydrochloric gallium gas. Then, an ammonia gas is introduced into the reactor and reacted with the hydrochloric gallium gas, to deposit and fabricate the GaN film on the substrate. The HVPE method has a higher film growth rate than a MOCVD method or a MOVPE method. For example, in the MOVPE method, a GaN film can be epitaxially grown typically at only several xcexcm/hour, but in the HVPE method, the GaN film can be epitaxially grown typically at several hundreds xcexcm/hour. Therefore, the HVPE method has its advantage in forming a thicker III-V nitride film.
However, a good quality AlxGayInzN film can not be provided by the HVPE method, and the fluctuation in thickness on the same substrate may be increased. On the other hand, forming the AlxGayInzN film via the MOVPE method, is a time consuming process and thus, the fabrication cost of the AlxGayInzN film is relatively high.
In the case of making an AlxGayInzN (x+y+z=1) film using a MOCVD method, a given substrate is set and held on a susceptor installed in a reactor, and is heated to a predetermined temperature by a heater. Then, a trimethylaluminun gas, a trimethylgallium gas, a trimethylindium gas or the like as III raw material gases are introduced with a carrier gas composed of a hydrogen gas or a nitrogen gas into the reactor. An ammonia gas as a V raw material gas is introduced with a carrier gas composed of a hydrogen gas or a nitrogen gas into the reactor. Then, the III raw material gases and the V raw material gas are reacted, to deposit and form the AlxGayInzN film on the substrate. As the AlxGayInzN film, an aluminum nitride film, a gallium nitride film, an indium nitride film, an aluminum-gallium nitride film, an aluminum-indium nitride film and a gallium-indium nitride film are exemplified.
In the above conventional method such as a MOCVD method, if the reaction between the III raw material gases and the V raw material gas is created on the wall surfaces of the reactor, the film-forming efficiency is degraded, and thus, the film growth rate is decreased. In the past, therefore, although it is considered that a cooling jacket is attached to the reactor, it is very difficult to directly attach the cooling jacket to the reactor because the reactor is made of quartz into a complicated figuration. As a result, the reactor is covered with a stainless tube on which a cooling jacket is attached.
In such a conventional fabricating apparatus, since the reactor is only indirectly cooled by the cooling jacket, it can not be cooled down sufficiently. Particularly, it was confirmed that an AlN film or an Al-rich AlxGayInzN (x+y+z=1, x greater than 0.5, yxe2x89xa70, Zxe2x89xa70) film can not be fabricated sufficiently, as compared with a GaN film or an Al-poor AlxGayInzN (x+y+z=1, 0xe2x89xa6x less than 0.5, yxe2x89xa70, Zxe2x89xa70) film. The reason is because in fabricating such an Al-rich AlxGayInzN film, a large amount of trimethylaluminum and a large amount of ammonia are employed as raw material gases and thus, the large proportions of the raw material gases are reacted on the wall of the reactor and on the susceptor heated to a higher temperature. As a result, the epitaxial growth of the Al-rich AlxGayInzN (x+y+z=1, x greater than 0.5, yxe2x89xa70, Zxe2x89xa70) film is inhibited.
Particularly, in the case that the susceptor is set on the bottom wall of the reactor and the substrate is set on the susceptor so that the main surface of the substrate is faced to the top wall of the reactor, a large amount of aluminum nitrides may be deposited on the top wall because the top wall is easy to be heated to a high temperature due to the radiant heat from the susceptor. Since the aluminum nitrides may be broken away from on the top wall and introduced into the growing Al-rich AlxGayInzN film, the crystal quality of the Al-rich AlxGayInzN film may be deteriorated.
In light of the above-mentioned problems, such a technique as to cool down the raw material gases with the cooling jackets attached to the nozzles to introduce the raw material gases into the reactor is disclosed in the Japanese Laid-open Publications Kokai Hei 10-167883 (JP A 10-167883) and Kokai Hei 10-67884 (JP A 10-67884). Moreover, such a technique as to cool down the raw material gases around the susceptor with a cooling jacket provided on the upper side from the susceptor is disclosed in the Japanese Laid-open Publication Kokai Hei 10-100726 (JP A 10-100726).
According to such a conventional technique, although the film growth rate and the crystal quality of the Al-rich AlxGayInzN film can be improved to some degree, they are not sufficient. Particularly, since the portion of the top wall of the reactor opposing to the substrate on the susceptor is not cooled down sufficiently, the broken away aluminum nitrides may deteriorate the crystal quality of the Al-rich AlxGayInzN film to large degree.
In addition, when using the conventional technique, the fluctuation in thickness of the AlxGayInzN film is increased. Particularly, when employing a larger substrate such as a 3-inch wafer, the fluctuation in thickness becomes conspicuous.
Moreover, it is proposed that a vertical reactor tube is employed and a substrate is set vertically in the reactor tube, that is, substantially parallel to the wall of the reactor tube. In this case, although raw material gases are introduced into the reactor tube from the top via nozzles cooled down with a cooling jacket, the nozzles may be stopped up through the reaction between the raw material gases in the nozzles.
It is an object of the present invention to work out the above conventional problems, and thus, to provide a method for epitaxially growing a good quality AlxGayInzN (x+y+z=1, xxe2x89xa70, yxe2x89xa70, Zxe2x89xa70)film at a higher film growth rate without the fluctuation in thickness using a MOCVD method.
It is another object of the present invention to provide an apparatus for epitaxially growing a good quality AlxGayInzN (x+y+z=1, xxe2x89xa70, yxe2x89xa70, Zxe2x89xa70) film at a higher film growth rate without the fluctuation in thickness via a MOCVD method.
In order to achieve the above object, this invention relates to a method for fabricating a III-V nitride film, including the steps of preparing a reactor horizontally, setting a substrate onto a susceptor installed in the reactor, heating the substrate to a predetermined temperature, directly cooling at least the portion of the inner wall of the reactor opposite to the substrate, and introducing a III raw material gas and a V raw material gas with a carrier gas onto the substrate, and thus, fabricating a III-V nitride film by a MOCVD method.
The effect of the present invention can be exhibited in the case that in forming the III-V nitride film, the susceptor is set on the bottom wall of the reactor so as to oppose the top wall of the reactor, and the substrate is set on the susceptor so that the main surface of the substrate is opposed to the top wall of the reactor. However, the susceptor may be set on the top wall of the reactor, and the substrate is set on the susceptor so that the main surface of the substrate is opposed to the bottom wall of the reactor.
According to the present invention, a large amount of trimethylaluminum and a large amount of ammonia are introduced into the reactor as raw material gases, to be able to fabricate an Al-rich AlGaInN (x+y+z=1, x greater than 0.5, yxe2x89xa70, Zxe2x89xa70) film or an AlN film in good quality at a higher film growth rate by a MOCVD method.
In the present invention, the substrate may be made of oxide single crystal such as sapphire single crystal, ZnO single crystal, LiAlO2 single crystal, LiGaO2 single crystal, MgAl2O4 single crystal, or MgO single crystal, IV single crystal or IV-IV single crystal such as Si single crystal or SiC single crystal, III-V single crystal such as GaAs single crystal, AlN single crystal, GaN single crystal or AlGaN single crystal, and boride single crystal such as ZrB2. Moreover, the substrate may be made of an epitaxial substrate composed of a base material made of the above-mentioned single crystal and an epitaxial film made of oxide single crystal such as ZnO single crystal or MgO single crystal, IV single crystal or IV-IV single crystal such as Si single crystal or SiC single crystal, III-V single crystal such as GaAs single crystal, InP single crystal, AlN single crystal, GaN single crystal or AlGaN single crystal, and boride single crystal such as ZrB2.
This invention also relates to an apparatus for fabricating a III-V nitride film using a MOCVD method, including a reactor prepared horizontally, a susceptor to hold a substrate thereon installed in the reactor, a heater to heat the substrate to a predetermined temperature via the susceptor, and a cooling means to directly cool down at least the portion of the inner wall of the reactor opposite to the substrate.
In a preferred embodiment of the fabricating apparatus of the present invention, the portions of the inner wall of the reactor opposite to the susceptor and in the upper stream from the substrate of the raw material gases are cooled down with the cooling means. In this case, two cooling jackets may be prepared for the portions opposite to the susceptor and in the upper stream from the substrate. The cooling jacket may be made of stainless steel. As a result, the configuration of the apparatus is simplified, and thus, the cost of the apparatus can be lowered
In another preferred embodiment of the fabricating apparatus of the present invention, the cooling means includes a cooling jacket directly attached to or built in the inner wall of the reactor, a pump to circulate a cooling medium through the cooling jacket and a cooling medium temperature-controlling instrument. As the cooling medium, water may be exemplified.
In still another preferred embodiment of the fabricating apparatus of the present invention, the reactor with the cooling means is covered with a housing entirely, and another cooling means is provided on the outer side of the housing. In this case, the whole of the reactor can be cooled down effectively.
In a further preferred embodiment of the fabricating apparatus of the present invention, the reactor is made of stainless steel entirely, and the whole of the reactor is cooled down directly with the cooling means. In this case, the configuration of the reactor can be simplified, and thus, the fabricating cost of a III-V nitride film can be reduced.